The present invention relates to an aqueous ink composition suitable for use as printing ink and stamping ink in general use, and for use with writing instruments, recording apparatus or the like. More particularly, the present invention relates to an aqueous ink composition for ink-jet printing, which is capable of yielding printed images with high quality (i.e., high clearness and sharpness, etc.), and with excellent resistance to water and light, and which is also capable of providing stable ink ejection, without causing the plugging of the nozzles of the ink-jet printing apparatus, even if it is used continuously for a long period of time or it is used intermittently after a period of non-use of the ink-jet printing apparatus.
Generally, an aqueous ink comprises as the main components a dye, water and a humectant consisting of a polyhydric alcohol or an ether thereof. In particular, an aqueous ink for use in ink-jet printing should satisfy the following conditions in order to yield an excellent print:
First, in order that the ink be compatible with ink droplet formation and control of the direction of the ejected ink droplet streams, it is necessary that its viscosity, surface tension, specific electric conductivity, and density each fall within certain appropriate ranges.
Second, it is necessary that no precipitates be separated, due to coagulation, precipitation, chemical change of slightly soluble components or other causes, from the ink during an extended period of continuous use or storage, or during the periods when the apparatus is not in use. Nor should the physical properties of the ink otherwise change during the above-mentioned periods. If the separated solid components or viscous materials in the ink become affixed around the nozzles, or if the physical properties of the ink change, deviating from the predetermined physical properties achieved at the time of the preparation, the desired printing quality, the desired ink ejection stability, and the desired ink ejection response cannot be obtained.
Third, it is desired that the ink provide adequately high contrast and clearness in the printed image.
Fourth, it is necessary that the ink for ink-jet printing be such that the printed images are resistant to water, light and physical wear.
Fifth, it is necessary that the images printed by the ink dry rapidly.
So far, many proposals have been made for an ink for ink-jet printing, but from the viewpoint of practical use, a satisfactory ink which is capable of meeting the above-described requirements has not been found.
As the dyes for use in the conventional black aqueous inks, for example, direct dyes such as C.I. Direct Black -2, -4, -17, -19, -22, -32, -38, -51, -56, -62, -71, -74, -75, -77, -105, -108, -112 and -154; and acid dyes such as C.I. Acid Black -1, -2, -7, -24 and -94 are employed in practice.
As the dyes for use in the conventional cyan inks, for example, direct dyes such as C.I. Direct Blue -1, -8, -71, -76, -78, -108, -163, -195, -202, and -236; and acid dyes such as C.I. Acid Blue -1, -7, -126, -175, and -234 are employed.
As the dyes for use in the conventional magenta inks, for example, direct dyes such as C.I. Direct Red -1, -11, -37, -62, -75, -83, -99, -220 and -227; acid dyes such as C.I. Acid Red -87, -92, -94, -115, -131, -154, -186 and -254 are employed.
As the dyes for use in the conventional yellow inks, for example, direct dyes such as C.I. Direct Yellow -12, -27, -33, -39, -50, -58, -85, -86, -88, -100 and -110; and acid dyes such as C.I. Acid Yellow -7, -17, -23, -29, -42, and -99 are employed.
Of the above-mentioned dyes for the conventional aqueous inks, the solubility of the direct dyes in the solvent employed in the inks is so low that the concentration of the dye cannot be increased. The result is that the ink cannot provide images with high density and high contrast. Furthermore, even if the concentration of the direct dye in the ink is held down, the dye precipitates from the ink during an extend period of storage, or during the periods when the ink-jet printing apparatus is not in use. As a result, the nozzles are plugged with the precipitates and it becomes impossble to obtain the desired ink ejection stability and the desired ink ejection response.
In order to make improvement on the above-described shortcomings of the conventional aqueous inks, there have been proposed a method of adding a dye-dissolving agent such as dimethylformamide, pyrrolidone, ethanolamine or a surface active agent, or a method of adding a large amount of a humectant to the ink. These agents, when added, bring about the following problems. The amines, for example, corrode the metallic materials employed in the ink-jet printing apparatus. By the addition of a surface active agent, foams are formed in the ink, by which the quality of printed images is decreased. When a large amount of a humectant is contained in the ink, the drying speed of the printed images is significantly decreased, and special paper with high ink-absorbing properties has to be employed in order to overcome this shortcoming.
Furthermore, the conventional azo dyes, which are polyazo dyes, have the shortcoming that they are contaminated with impurities, such as diazonium decomposition materials, diazonium amines, unreacted monazonium compounds and disazo compounds, which are contained in the azo dyes in the course of a number of reaction steps involved in the production of the polyazo dyes.
Aqueous inks employing the conventional acid dyes provide images with clear color tone. However, images printed by the aqueous inks are not resistant to water, and special paper has to be employed in order to eliminate this shortcoming.
In order to eliminate the above-described shortcomings, and in order to obtain an aqueous ink which satisfies the previously described conditions, there has been a keen demand for a dye which has excellent solubility in the solvent of the ink, and is capable of yielding printed images which are resistant to water, light and physical wear, and in which substantially no impurities are contained.